Here's their demo but with some color added: http://codepen.io/anon/pen/VYQMea
But yeah, you're typically stuck with one color. Here's a demo that shows off multi-color fonts (Firefox only): http://pixelambacht.nl/2014/compyx-a-multicolor-8bit-font/
That technique could be used to make multi-color font icons if it were cross-browser compatible. Maybe one day. But I think SVG icons are where things are headed.
As for compatibility issues, there's one listed on this page: https://news.ycombinator.com/item?id=9047846
Here's their demo but with some color added: http://codepen.io/anon/pen/VYQMea
But yeah, you're typically stuck with one color. Here's a demo that shows off multi-color fonts (Firefox only): http://pixelambacht.nl/2014/compyx-a-multicolor-8bit-font/
That technique could be used to make multi-color font icons if it were cross-browser compatible. Maybe one day. But I think SVG icons are where things are headed.
As for compatibility issues, there's one listed on this page: https://news.ycombinator.com/item?id=9047846